I. Technical Field
The present invention relates to a separator for a fuel cell, a method for producing the separator, and a solid oxide fuel cell (SOFC).
II. Description of the Related Art
As is well known, the solid oxide fuel cell has been investigated and developed as one of various kinds of fuel cells for power generation. As the structure of the solid oxide fuel cell, there are proposed at present three types of structures: a cylindrical type, a monolithic type, and a flat plate laminated type. However, the flat plate laminated type structure is widely adopted for low-temperature operating type solid oxide fuel cell.
In the flat plate laminated type solid oxide fuel cell, a fuel cell stack is constituted by alternately laminating power generation cells and separators in a state where a current collector is sandwiched between the power generation cell and the separator. The power generation cell has a laminated structure in which a solid electrolyte layer made of an oxide ion conductor is sandwiched between an air electrode (cathode) layer and a fuel electrode (anode) layer. Oxygen (air) as oxidizer gas is supplied to the air electrode side of the power generation cell, while fuel gas (H2, CH4 and the like) is supplied to the fuel electrode side. Both the air electrode and the fuel electrode are made porous so as to enable the oxygen and the fuel gas to reach their boundary with the solid electrolyte.
On the other hand, the separator has a function of electrically connecting the power generation cells to each other and of supplying reactive gas to the power generation cell, and is provided with a fuel passage which introduces fuel gas from an outer peripheral part and which discharges the fuel gas from a surface facing the fuel electrode layer, and with oxidizer passage which introduces air as oxidizer gas from an outer peripheral part and which discharges the air from a surface facing the air electrode layer. An air electrode current collector made of a sponge-like porous sintered metallic plate such as an Ag-based alloy is arranged between the separator and the air electrode of the power generation cell, and a fuel electrode current collector made of a sponge-like porous sintered metallic plate such as a Ni-based alloy is arranged between the separator and the fuel electrode of the power generation cell.
In the solid oxide fuel cell constituted as described above, the oxygen supplied to the air electrode side of the power generation cell via the separator and the air electrode current collector reaches near the boundary with the solid electrolyte through the pore in the air electrode layer, and there, the oxygen receives an electron from the air electrode to be ionized to oxide ion (O2−). The oxide ion is diffusively moved in the solid electrolyte toward the direction of the fuel electrode. When reaching near the boundary with the fuel electrode, the oxide ion reacts there with fuel gas to produce a reaction product (H2O and the like), and emits an electron to the fuel electrode. The electron is taken out by the fuel electrode current collector, and thereby current is made to flow and a predetermined electromotive force can be obtained.
Meanwhile, the solid oxide fuel cell of this type includes a solid oxide fuel cell having a seal-less structure to eliminate a gas leakage prevention seal (for which a glass seal is conventionally used) in the outer peripheral part of the power generation cell. The solid oxide fuel cell having the seal-less structure is arranged in such a manner that discharge ports of fuel gas and oxidizer gas (reactive gas) are provided in the center part of the separator, that while the reactive gas discharged from the discharge port is made to diffuse in the outer peripheral direction of the power generation cell, the reactive gas is made to spread in excellent distribution over a whole part of the fuel electrode layer and the air electrode layer so as to generate power generation reaction, and that the gas generated by the power generation reaction and the remaining gas not used in the power generation reaction is exhausted from the outer peripheral part of the power generation cell to the outside (for example, see Japanese Patent Laid-Open No. 11-016581).
However, in the solid oxide fuel cell having the above described seal-less structure, when the reactive gas discharge port is provided in the center part of the separator, there occurs a phenomenon in which gas concentration with a peak at the center part near the discharge port is reduced toward the outer peripheral part. This results in a problem that electrode reaction may not be uniform on the cell face to cause deviation of current density distribution within the cell face and thereby the efficiency of the power generation cell (output power density per unit area) may be remarkably decreased. Further, the electrode reaction as exothermic reaction which is not uniformly performed within the cell face, causes a temperature gradient to be formed in the power generation cell, as a result of which the power generation cell may be damaged by thermal stress in the course of the reaction.
On the other hand, in the flat plate laminated type solid oxide fuel cell, when the operating temperature is set to a low temperature not higher than 800° C., a separator made of stainless steel is often adopted as the above described separator.
However, the fuel cell in which the separator made of stainless steel is adopted, has a problem that when a hydrocarbon compound, such as methane, is used as fuel gas, carbon and carbon oxide are generated by reforming reaction, so that a part exposed to the fuel gas, such as the fuel passage wall surface of the separator, is carburized by the reaction product and thereby the separator deteriorates in the early stage.